Miocene drainage reversal of the Amazon River driven by plate--mantle interaction

Northern South America experienced significant changes in drainage patterns during the opening of the South Atlantic Ocean. Disappearance of a mega-wetland in the western Amazonian basins was followed by the formation of the eastward-draining Amazon River, which has been attributed to Andean uplift1–5. However, South America’s westward motion over cold, dense subducted slabs implies that regional subsidence and uplift east of the Andes may have been driven by mantle convection. Here we use a coupled model of mantle convection and plate kinematics to show that dynamic subsidence of up to 40 m Myr−1 initially formed the Amazonian mega-wetland. In our model, the sustained westward motion of continental South America over subducted slabs resulted in rebound of the Amazonian mega-wetland region at rates of up to 40 m Myr−1 after 30 million years ago, paired with continued subsidence of the eastern Amazonian sedimentary basins at 10–20 m Myr−1. The resulting progressive tilt of northern South America to the east enabled the establishment of the Amazon River, suggesting that mantle convection can profoundly affect the evolution of continental drainage systems. During the Early Miocene (from ∼23Myr bp), an inland fluviolacustrine/marginally marine Amazonian system partially flooded northwest South America with regional drainage mainly northwards towards the Caribbean1–4. This Amazonian megawetland existed at least from the Middle to Late Miocene1,2 when the dominant fluvial drainage switched to its present course towards the Atlantic. This shift, initiating the fluviolacustrine system that is now the Amazon River is supported by sediment provenance, palaeo-transport direction and biostratigraphic studies1–3,5. The timing of this onset is estimated from offshore sedimentary records between 10.6 and 9.7Myr bp (ref. 6). Any possible Amazonian– Caribbean connection was fully closed by the Late Miocene2, with the Amazon River reaching its present shape and size from the Pleistocene onwards2,7 (∼2.4Myr bp). During the Early andMiddle Miocene a restricted eastward-flowing palaeo-Amazon River may have drained to the Atlantic coast7. Stratigraphic data suggest that this fluvial system extended westward until limited by the Purus arch7 (Fig. 1), a structural high that separated the Solimões and Amazonas basins and restricted the Amazonian mega-wetland to the west and the palaeo-Amazon River to the east. The main Miocene changes in drainage patterns, and the formation of the Amazon River, are commonly attributed to uplift in the northwest Andes5,8–11. It has been proposed that the uplift of the Eastern Cordillera caused flexural subsidence in the foreland basins, and redirected a significant portion of the northwest Amazonas basin drainage, along the northward-flowing Palaeo-Orinoco and Magdalena rivers, to the east2. In addition, it has also been suggested that Amazon landscape evolution can